In UMTS LTE (Universal Mobile Telecommunication Standard-Long Term Evolution), a standard currently under development and advocated by 3GPP (3rd Generation Partnership Project), it is proposed that downlink (DL) control information (DCI) be encoded by a rate ⅓ tail-biting (TB) convolutional code.
A tail-biting code imposes the tail-biting criterion, which forces the convolutional encoder to start and end in the same state. However, in contrast to the traditional use of a convolutional encoder, any state may be used as start and end states. In spite of the use of a convolutional encoder for implementation, a tail-biting code is, in fact, a block code. For feed forward encoders, such as the one used in UMTS LTE, the tail-biting criterion may be achieved by loading the registers of the encoder with the last m bits of the K-bit information block to be input to the encoder, where m is the encoder memory.
In UMTS LTE, the downlink control information is made up by an aggregation of control channel elements (CCE). The number of aggregated control channel elements is denoted s and belongs to an ensemble S. Each CCE is in turn made up by a number (X) of resource elements (RE), as specified in 3GPP TS 36.211 V8.2.0 (2008-03).
Various parameters of the encoding process, such as the number s and the length of the trellis associated with the tail-biting code, may not be known to the decoder. Therefore, the decoder needs to perform blind detection, i.e. test different hypotheses regarding the encoding parameters, to decode the information correctly. The number of hypotheses that need to be tested may be large. Thus, the decoding effort may be substantial.
Therefore, there is a need for improved methods and arrangements for decoding a signal encoded by a tail-biting code.
An overview of tail-biting codes may be found in H. H. Ma and J. K. Wolf, “On tail-biting convolutional codes,” IEEE Trans. Commun., vol. 34, no. 2, pp. 104-111, February 1986.
Example algorithms suitable for decoding of tail-biting codes may be found in R. Y. Shao, S. Lin and M. P. Fossorier, “Two decoding algorithms for tail-biting codes,” IEEE Trans. Commun., vol. 51, no. 10, pp. 1658-1665, October 2003, in J. B. Anderson and S. M. Hladik, “A circular Viterbi algorithm for the bounded distance criterion”, IEEE Trans. Commun., vol. 50, no. 11, pp. 1736-1742, November 2002, and in J. B. Anderson and S. M. Hladik, “Tailbiting MAP decoders,” IEEE J. Select. Areas Commun., vol. 16, no. 2, pp. 297-302, February 1998.
A UMTS LTE standard proposal regarding the downlink control information format may be found in R1-071683, Nokia, “Tree structure for the DL control channel,” 3GPP RAN1#48bis, St. Julians, Malta, March 2007.